This invention relates to a recording sheet for printing having a coating layer which comprises a polyvinyl alcohol resin (PVA) as a resin component and which has an improved water resistance, a reduced degree of blotting and good ink absorbency, and enables a highly clear print. The recording sheet is especially adapted for ink jet recording.
For recording mediums for various printings and recordings including gravure, letterpress, ink jet printing and recordings, it is usual to use ordinary paper. As high performances in printing and recording techniques, such as a high printing and recording speed and multicolor recording or printing, are now in progress, higher characteristic properties are required of printing and recording paper.
Especially, in the ink jet recording system, ink droplets are produced according to an ink jetting system such as an electrostatic suction system using application of high voltage or a system wherein an ink is mechanically vibrated or displaced by means of a piezoelectric element. The droplets are jetted against a material to be recorded, e.g. paper, and deposited thereon as recorded. Since this system involves a reduced occurrence of noises, is easy in coloration, and ensures high-speed recording and printing, it has been widely applied to various types of printers. In recent years, the ink jet recording machine is developed to realize high speed and multicolor operations. This, in turn, strongly requires a higher quality of ink jet recording paper.
More particularly, the ink jet recording paper should have such characteristics: (1) an absorption speed of ink is high; (2) ink dots have a diameter which is not larger than required (not blotted); (3) when ink dots are superposed, an ink dot or dots deposited later are not run out over the dots deposited beforehand; and (4) an ink jet recording sheet has a good water resistance.
Extensive studies have been now made in order to improve printability or recording properties by coating an absorbing material for water-soluble ink jet recording ink, e.g. paper, or a sheet or film of a plastic material such as polyethylene terephthalate, polyethylene, polypropylene or the like, with a water-soluble resin such as a polyvinyl alcohol (PVA) resin. However, there arises the problem that blotting of an ink on the absorbing material causes a diameter of an ink dot to be larger than required, with a loss of the clarity, and a water resistance lowers considerably.
On the other hand, the ink jet recording system may be used such that transparent films made of various types of plastics including polyethylene terephthalate (PET), polypropylene, polyethylene and nylons are provided as a material to be recorded and are directly printed thereon for use as OHP sheets or for observation of transmitted light through color displays.
In this case, however, the absorption speed of a water-soluble ink is not satisfactory. If a water-soluble resin is coated and a printed film is immersed in water, not only the water-soluble resin is dissolved out, but also intimate contact between the film and the water-soluble resin cannot be obtained, thereby presenting the problem that the resin in whole is separated from the film.
In order to solve the above problem, an attempt has been made to appropriately control the degree of saponification of PVA resin which is a water-soluble resin, thereby improving the water resistance through hydrogen bond after coating of the film. Alternatively, other attempts have been made so as to improve the water resistance, in which PVA resin is imparted with a cationic or anionic functional group as described in Japanese Laid-open Patent Application No. 63-183874, and in which a crosslinking agent is used so that functional groups are crosslinked.
However, the PVA resin, which has a controlled degree of saponification or which has cationic or anionic functional groups, cannot be crosslinked to a satisfactory extent. Thus, adequate water-resistant properties cannot be imparted thereto.
On the other hand, in the methods of using a crosslinking agent, it is usual to use, as such a crosslinking agent, aziridine compounds, epoxy compounds, blocked isocyanate compounds, oxazoline compounds and the like. The method of using oxazoline compounds and/or blocked isocyanate compounds among them has the problem that a curing temperature is as high as 80 to 180.degree. C., so that an expensive equipment is necessary and a substrate to be coated having a low heat resistance cannot be employed. Especially, blocked isocyanate compounds may adversely influence environment and the body of a worker because a blocking agent used to block isocyanate groups is volatilized upon curing.
The crosslinking agents made of aziridine compounds and/or epoxy compounds are disadvantageous in that since these compounds are so low storage stability that when stored under high temperature and high humidity conditions, they are polymerized and hardened, and that they have very strong toxicity, and greatest care must be paid to handling.
Moreover, conventional crosslinking agents are not satisfactory with respect to reactivity at low temperatures, and are not sufficiently miscible with water-soluble resins to be crosslinked. Thus, a desirable water resistance cannot be imparted to water-soluble resins.